The ability to “virgin birth” discovered in crocodiles for the first time ever

For the first time, scientists have found evidence that female crocodiles can lay eggs without mating, using a strange reproductive strategy that may have its evolutionary roots in the age of the dinosaurs.

In 2018, a lone female American alligator (Acute Crocodylus) kept in captivity for 16 years laid a clutch of eggs, with one containing a discernible fetus, a female like its mother.

Genetic analyzes by a team of US scientists have now revealed that the crocodile produced the eggs without any input from a male mate, in a process called parthenogenesis, more commonly known as ‘virgin births’.

While the eggs didn’t hatch, it’s a startling discovery in a new branch of the animal kingdom that shows how far this unusual reproductive strategy dates back.

Now that virgin births have been documented in crocodiles and birds, the findings suggest that their ancient ancestors, the dinosaurs, may have shared these miraculous reproductive abilities.

Crocodiles and birds are living members of a clade of reptiles called archosaurs which once, if we trace its branches, also included dinosaurs and flying reptiles.

“[T]its discovery offers tantalizing insights into the possible reproductive capabilities of extinct archosaur relatives of crocodilians and birds, particularly members of Pterosauria and Dinosauria,” writes the team of researchers led by evolutionary biologist Warren Booth of Virginia Tech.

Parthenogenesis is a form of asexual reproduction in which female animals that would normally need a male’s sperm to reproduce can do so without mating. Rather than storing sperm for years, as reptiles can, in a pinch, females can fuse two of their cells to create a viable embryo that has only one parent.

Once considered rare, scientists slowly realized that parthenogenesis was more common in vertebrate animals than initially thought.

Plants and invertebrates have been working on it for a while, but it took some time for researchers to realize that vertebrate females could produce offspring from eggs unfertilized by male sperm.

Virgin births have since been observed in more than 80 vertebrate species, including lizards, snakes, sharks and rays, but mostly in captive animals and never before documented outside those vertebrate lineages until now.

Sure enough, when the scientists looked even closer, they started finding examples of facultative parthenogenesis in wild animals, following the hunch that it could be a survival strategy that females adopt when they can’t find a male mate, especially in scattered populations on the verge of extinction.

However, more recent discoveries of captive-bred California condors have suggested that these critically endangered birds can reproduce independently even when females are in regular contact with perfectly fertile males, so go figure.

As for the crocodile, genetic analysis of the stillborn relative to the mother showed that they shared virtually identical genotypes.

This clone-like resemblance suggests that terminal fusion automixis was the reproductive mechanism in this example of parthenogenesis, just as it has been observed in studies of birds, snakes, and lizards.

Phylogenetic tree showing the evolutionary relationship between snakes, lizards, crocodiles and birds.
Parthenogenesis has been observed in lizards, snakes, birds, and now crocodiles. (Boot et al., biology letters, 2023)

This, Booth and colleagues write, indicates that parthenogenesis is a trait “likely possessed by a distant common ancestor of these lineages.” However, more research is needed to “thoroughly test the distribution and evolutionary dynamics of [facultative parthenogenesis] through a deeper evolutionary time.”

Terminal fusion automixis is one way virgin births occur: A female fuses an egg that contains half of her chromosomes with another type of haploid cell called polar bodies that are left over from the ovaries’ normal egg production. It involves a slight shuffling of genetic material to fill in the missing sperm blanks, and the resulting offspring are nearly clones of their mothers.

So while it might be a way for females to reproduce on their own when mates are few and far between, it’s not a sustainable way to produce offspring because they lack the genetic diversity that two parents carry.

And while the eggs didn’t hatch in this case, the researchers say that doesn’t rule out the possibility that crocodiles use parthenogenesis to produce viable offspring. Rates of parthenogenetic eggs hatching in other species can be as high as 3%.

The study was published in Biology Letters.

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